An investigation of the RPC-5 profiles of the lysine transfer RNA population of a variety of tissue culture cell lines has shown that a correlation exists between the concentration of lys-tRNA4 and the rate of cell division. We are investigating whether the synthesis of this lys-tRNA species represents an important mechanism controlling the commitment of eukaryotic cells to begin traverse of the cell cycle. Further it is the aim of this research to establish the biosynthetic pathway for the synthesis of lys-tRNA4, the structure of the intermediate tRNA species and the tRNA modification enzymes involved in each step. It is proposed that modulation of these enzymes by factors in the cells environment controls the levels of lys-tRNA4 and therefore the growth rate of cells. This theory will be tested using in vitro enzyme assay systems and in vivo tRNA processing studies.